1. Field of the Invention
The present invention relates to catalyst precursors, to methods of making the catalyst precursors and to methods of using the catalyst precursors. The catalyst precursors of the present invention are useful in preparing .alpha.-olefin polymerization procatalysts that can yield polymerized .alpha.-olefins having a narrow polymer size distribution and reduced polymer fines. The catalyst precursors also are easier to handle, transport to and from downstream processes, and produce polymers having improved extruder operability, less filter clogging, less "talc" (or polymer fine particles &lt;10 micron) and reduced polymer product segregation in storage.
2. Description of Related Art
Polymers and copolymers of lower .alpha.-olefins, particularly, ethylene, propylene and butylene are widely used throughout the world. These polymeric products typically are relatively inexpensive to manufacture, and they exhibit a number of commercially useful properties. When ethylene is polymerized, the process is relatively uncomplicated in that the product type is not influenced by the manner in which the ethylene molecules add to the growing polymeric chain during polymerization. The polymeric product of ethylene does not generally exist in stereoisomeric forms.
When propylene is polymerized, however, the presence of pendant methyl groups on the polymeric chain provides a possibility of several product types, depending on the steric regularity with which propylene molecules add to the growing chain. Much if not most of the commercial polypropylene results from the stereoregular addition of propylene molecules in a regular head-to-tail manner. The form of polymer having a substantial proportion of random addition of propylene units is termed atactic and this amorphous form is less desirable. If present in a significant proportion, the atactic polymer must be removed through an extraction process to provide a more desirable crystalline material.
These polymers typically are formed by using a polymerization catalyst. The activity of the catalyst is significant in that the more polymer produced per unit weight of catalyst the better. The early titanium, chromium or vanadium catalysts were of low activity and the product contained a significant proportion of catalyst residues. These residues had to be removed in an effort to obtain commercially satisfactory properties.
More recent titanium-based olefin polymerization catalysts are stereoregulating and have sufficient activity to avoid extraction and deashing. These high activity catalysts typically are prepared from a solid procatalyst that usually contains magnesium, titanium and halide moieties, a cocatalyst (usually an organoaluminum compound) and a selectivity control agent (SCA). The solid titanium-containing compound typically is referred to as a "procatalyst," the organoaluminum compound, whether complexed or not, usually is referred to as the "cocatalyst" and the third component external electron donor, whether used separately or partially or totally complexed with the organoaluminum compound, is referred to as the "selectivity control agent." Throughout this disclosure, these terms will be used in accordance with the aforementioned designations.
Many chemical combinations of procatalysts, cocatalysts and selectivity control agents are known in the art to produce active catalysts. Through considerable experience, however, certain materials are of greater interest than others. For example, there is significant research in the area of procatalysts, which typically is some chemical combination of magnesium, titanium tetrachloride and an internal electron donor. These internal electron donors usually are aromatic esters such as ethyl benzoate or ethyl p-toluate. Conventional cocatalysts include an aluminum trialkyl such as triethylaluminum or triisobutylaluminum that is often complexed with a portion of the selectivity control agent (or external electron donor), which also is typically an aromatic ester. Although variations in any of these catalyst components will influence the performance of the resultant catalyst, the component that appears to offer the greatest opportunity for modification to produce greater catalyst activity is the procatalyst.
The literature is rife with disclosures relating to the various known methods of preparing procatalysts. For example, Kioka, et al., U.S. Pat. No. 4,330,649, the disclosure of which is incorporated by reference herein in its entirety, describes a solid catalyst component (procatalyst) that is prepared by heating a soluble magnesium compound such as magnesium chloride with a higher alcohol in the presence of an ester to produce a solution. This solution contains a "precursor" of the procatalyst, which then is added to titanium tetrachloride and an electron donor (internal) to form the procatalyst. Brand, U.S. Pat. No. 4,472,521, the disclosure of which is incorporated by reference herein in its entirety, reacts a magnesium alkoxide, wherein each alkoxide has four or more carbons, in the presence of an aromatic hydrocarbon. Titanium tetrachloride and an internal electron donor then are added to the resulting solution to form a solid procatalyst. Arzoumanidis, U.S. Pat. No. 4,540,679, the disclosure of which is incorporated by reference herein in its entirety, produces an olefin polymerization catalyst component by contacting a suspension of magnesium ethoxide in ethanol with carbon dioxide. The addition of organoaluminum in hydrocarbon results in the formation of granular particles that are employed as a support for a titanium compound upon contact with titanium tetrachloride. Nestlerode, et al., U.S. Pat. No. 4,728,705, the disclosure of which is incorporated by reference herein in its entirety, solubilizes magnesium ethoxide in ethanol with carbon dioxide and spray dries the resulting solution or uses the solution to impregnate carrier particles. The solid particles resulting from either modification are useful in the production of a procatalyst of desirable morphology.
A number of United States patents issued to Robert C. Job (and Robert C. Job, et al.,) describe various mechanisms for preparing magnesium-containing, titanium-containing compounds that are useful as precursors for the production of procatalysts that are ultimately useful in preparing catalysts for the polymerization of .alpha.-olefins. For example, U.S. Pat. Nos. 5,034,361; 5,082,907; 5,151,399; 5,229,342; 5,106,806; 5,146,028; 5,066,737; and 5,077,357, the disclosures of which are incorporated by reference herein in their entirety, disclose various procatalyst precursors. U.S. Pat. No. 5,034,361 discloses solubilizing a magnesium alkoxide in an alkanol solvent by interaction of the magnesium alkoxide compound and certain acidic materials. This magnesium alkoxide then can be used either directly as a magnesium-containing catalyst precursor, or can be reacted with various titanium compounds to produce a magnesium and titanium-containing catalysts precursor.
U.S. Pat. Nos. 5,082,907; 5,151,399; 5,229,342; 5,106,806; 5,146,028; 5,066,737; and 5,077,357 disclose various magnesium and titanium-containing catalyst precursors, some of which are prepared by using the aforementioned magnesium alkoxide as a starting material. These precursors are not active polymerization catalysts, and they do not contain any effective amounts of electron donor. Rather, the precursors are used as starting materials in a subsequent conversion to an active procatalyst. Magnesium and titanium-containing procatalysts are formed by reacting the magnesium and titanium-containing precursor with a tetravalent titanium halide, an optional hydrocarbon and an electron donor. The resulting procatalyst solid then is separated from the reaction slurry (by filtration, precipitation, crystallization, and the like). These procatalysts are then converted to polymerization catalysts by reaction with, for example, an organoaluminum compound and a selectivity control agent. It was known to store procatalyst components in a mineral oil slurry as described in, for example, U.S. Pat. No. 4,548,915, the disclosure of which is incorporated by reference herein in its entirety.
It was known previously to dry the precursor in laboratory experiments in an effort to analyze the precursor. The Job patents referred to above describe drying the precursors under flowing nitrogen at ambient temperatures for less than two hours. These dried solid precursors then were converted to procatalysts by reacting the precursor with additional titanium halides and an internal electron donor. The resulting solid procatalysts then were separated from the slurry and used together with a cocatalyst and an optional selectivity control agent in the polymerization of olefins.
Previous experience with pro-catalyst and pro-catalyst precursor suggested that, for certain cases, heating magnesium and titanium-containing catalyst particles (procatalyst or precursors) above 50.degree. C. during a drying processes would significantly hamper catalyst productivity. Thus, precursors have heretofore been commercially produced with limited temperature drying. Previously, it was known that drying the precursor at ambient temperatures (up to about 46.degree. C.) could yield a substantially dry product without significantly decreasing catalyst productivity. Drying was effected by passing nitrogen at about 46.degree. C. through the precursor material, which contained solvents (i.e., isopentane, isooctane, etc.) that were used to wash the material.
While the above drying conditions offered a material which was substantially dry (free of residual solvent) by analysis, difficulties have arisen during conveying of dried precursor to downstream processes Specifically, difficulties have arisen during this conveying, which takes an extended period of time (typically about 2 hours or more). There are multiple interruptions in the process that could stem from bridging or clumping of the "dry" precursor. In addition, there still exists a need to improve the material's eventual polymer product characteristics, including improved extruder operability, reduced filter clogging, reduced "talc" (or very fine polymer particles) and reduced polymer product segregation in storage prior to extrusion and pelletizing.